The temporal orienting P3 effect to non-target stimuli: Does it reflect motor inhibition?

Biological Psychology 89 (2012) 433–443

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The temporal orienting P3 effect to non-target stimuli: Does it reflect motor inhibition? Kathrin Lange ∗ Institut für Experimentelle Psychologie, Heinrich Heine Universität Düsseldorf, Universitätsstraße 1, 40225 Düsseldorf, Germany

a r t i c l e

i n f o

Article history: Received 7 October 2011 Accepted 9 December 2011 Available online 21 December 2011 Keywords: Temporal orienting ERP N1 P3 Auditory Motor inhibition

a b s t r a c t Temporal orienting enhances early (N1) and late (P3) stages of auditory processing. However, the functional significance of these effects has not been settled yet. The present study tested a motor inhibition account on the temporal orienting P3 effect to non-target stimuli. A temporal cuing paradigm was used, where the level of motor preparation (high vs. low) was varied: If motor preparation is higher, more inhibition is necessary to withhold a response when a non-target is presented at the attended time point. Consequently, if the enhanced P3 to temporally attended non-targets reflected increased motor inhibition, higher motor preparation should further enhance the P3. Overall, temporal orienting enhanced both the N1 and the P3, thus replicating earlier findings. Moreover, the temporal orienting P3 effect was larger when motor preparation was higher. Inconsistent with the motor-inhibition account, however, the P3 to temporally attended non-targets did not differ as a function of motor preparation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In most everyday situations, our senses receive much more information than is relevant for our current needs and goals. Because it is not particularly efficient to process all those stimuli to their full extend, we typically choose certain stimuli for prioritized processing. Which stimuli we attend is influenced by our current goals and expectations. Attention research has mainly focused on the spatial orienting of attention. However, there is accumulating evidence, both at a behavioral and at a neural level, that perception and action can also be improved by orienting attention to specific moments in time (for reviews see Nobre, 2001; Nobre et al., 2007; Nobre and Coull, 2010). Responding to stimuli at attended time points has been shown to be faster (and sometimes also more accurate) than responding to stimuli at unattended time points (reaction times: e.g. Correa et al., 2004; Coull and Nobre, 1998; Lampar and Lange, 2011; Lange and Heil, 2008; Lange and Röder, 2006; Miniussi et al., 1999; accuracy: Correa et al., 2005). Event-related potential (ERP) studies have shown enhancements of the visual P1 (Correa et al., 2006a) and the auditory N1 on the one hand (Astheimer and Sanders, 2009; Lampar and Lange, 2011, Experiment 2; Lange et al., 2003, 2006; Lange and Röder, 2006; Röder et al., 2007; Sanders and Astheimer, 2008) and of the N2 (Griffin et al., 2002; Miniussi et al., 1999; Sanders and Astheimer, 2008) and the P3 on the other (Griffin et al.,

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2002; Lampar and Lange, 2011; Lange et al., 2003, 2006; Miniussi et al., 1999; Sanders and Astheimer, 2008). These findings provide evidence that temporal orienting modulates stimulus processing both at early and at late processing levels. However, the functional significance of these effects still needs to be elucidated. The present study focused on investigating the functional significance of the temporal orienting P3 effect. An enhanced P3 has been observed in two different temporal orienting paradigms, where probabilistic cuing or differences in task relevance, respectively, were used to manipulate attention. In the temporal probabilistic cuing paradigm (e.g. Miniussi et al., 1999) attentional orienting is triggered by a cue, which indicates when the target will most likely appear. The target appears either at the time point indicated by the cue (valid) or at the other time point (invalid). Participants respond to both validly and invalidly cued targets. The enhanced P3 for valid targets that has been observed in this paradigm has been related to improvements in decision- or response-related processes (Nobre, 2001). The second paradigm, in which attention is guided by differential task relevance, is based on the spatial selective attention paradigm first introduced by Hillyard et al. (1973). In the temporal version of this task (e.g. Lange et al., 2003), sound pairs are presented, which are separated by short (e.g. 600 ms) or long (e.g. 1200 ms) temporal intervals with equal probability. Attention is manipulated by asking participants in each block of trials to respond to the second sound (S2) only if it is presented at the ending of one of the intervals (short or long): This is the attended time point. Which time point is attended varies between blocks of trials. S2s presented at the other time point never require any

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K. Lange / Biological Psychology 89 (2012) 433–443

response. Therefore, these stimuli can be completely ignored and their time of appearance is completely unattended. Since there are no overt responses to unattended stimuli, attention effects can only be obtained at a neural level in this paradigm, e.g. by comparing ERPs. When comparing ERPs it is crucial to avoid any confounding with physical differences between conditions. However, responding to attended but not to unattended stimuli yields a confound with motor-related activity. To overcome this problem, two variants of S2 are presented both at the attended and at the unattended time point: Non-targets and targets. Responding is only required to attended but not unattended targets and never to non-targets. Thus, attention effects uncontaminated by differential response assignment can be obtained by comparing ERPs to physically identical attended and unattended non-targets. Using this paradigm, temporal attention has been found to enhance the P3 to both target (Lange et al., 2003; Sanders and Astheimer, 2008) and non-target stimuli (Lange et al., 2003, 2006; but see Sanders and Astheimer, 2008). The temporal orienting effect observed for targets can be easily reconciled with improvements in target detection or response preparation, as has been suggested earlier (Sanders and Astheimer, 2008; see also Nobre, 2001). By contrast, it is highly unlikely that these processes also underlie the enhanced P3 to non-target stimuli, because these stimuli were no targets that had to be detected and because they did not require an overt response that had to be selected or prepared. Thus, the functional significance of the temporal orienting P3 effect to non-targets remains an open question. 1.1. The temporal orienting P3-effect to non-targets Among the factors that are known to influence P3 amplitude is the amount of task-relevant information provided by a stimulus (Johnson and Donchin, 1978; Ruchkin and Sutton, 1978; Sutton et al., 1965, 1967; see also Johnson, 1986). We have suggested earlier that the temporal orienting P3-effect may be based on the different amounts of information conveyed at attended and unattended endings of short or long intervals (Lange et al., 2003). In the temporal orienting paradigm, the most relevant information (respond – yes or no?) is provided at the attended time point – thus resulting in a larger P3 to attended stimuli. For the unattended conditions, the information content depends on whether the stimulus is presented at the end of the short or at the end of the long interval. At the end of the short interval, the participant does not know, prior to stimulus delivery, whether a response-relevant stimulus will be presented in that trial. Thus, if a stimulus is presented at the unattended ending of the short interval, this provides information that a response will not be required in that trial. By contrast, in the unattended long condition, the participant is already certain prior to stimulus occurrence that no response-relevant stimulus will follow, as only stimuli presented at the end of the short interval are task-relevant in this condition. Thus, task-relevant information cannot be provided at this time point. Our previous findings are consistent with this idea: Unattended non-targets elicited a P3 only when presented at the end of the short interval but not at the end of the long interval (Lange et al., 2003).

Karlin et al., 1970; Kok, 1986; Pfefferbaum et al., 1985; Roberts et al., 1994; Simson et al., 1977). It has been related to inhibitory processes, e.g. the inhibition of a planned motor response (Bruin et al., 2001; Karlin et al., 1970; Roberts et al., 1994; Smith et al., 2007; Zordan et al., 2008) or the evaluation of the outcome of inhibition (Beste et al., 2008, 2010; Dimoska et al., 2003, 2006; Fallgatter et al., 2004; Roche et al., 2005). When comparing the P3 between nogo and go stimuli, a potential confound arises from differences in movement-related potentials between these conditions. Bruin and colleagues therefore tested the notion that the nogo P3 reflects motor inhibition by comparing only ERPs to nogo stimuli under different degrees of response preparation (Bruin et al., 2001; see also Smith et al., 2007). More specifically, they manipulated the specificity of a priori motor preparation by presenting a cue at the beginning of each trial, which informed the participants whether or not a response could be required (“go” or “nogo” priming). There were two basic variants of go priming: Specific or unspecific. In unspecific go priming, the cue indicated that a go stimulus could follow, but not which response was required by the stimulus. By contrast, in specific go priming, the cue indicated the response hand required by the upcoming stimulus (left or right). It was expected that the participants used the information provided by the cue to prepare their responses in advance. Thus, if the participants knew that a nogo stimulus followed (nogo priming), they would not prepare a response at all. In unspecific go priming, only unspecific preparation of a response was feasible, whereas in specific go priming, the correct response hand should be activated. More specific response preparation was expected to be stronger than less specific preparation. Since stronger response preparation should require stronger inhibition for nogo stimuli, inhibition triggered by nogo stimuli should be strongest for specific go priming and weakest for specific nogo priming. This was indeed the pattern observed for the nogo P3, with intermediate amplitudes for the P3 elicited in the unspecific go priming condition (for similar results see Smith et al., 2007). This result pattern may be regarded as evidence that the nogo P3 reflects response inhibition. However, the assumption that participants indeed prepared their response according to the cue needs to be verified (Smith et al., 2007). In particular, activation of the response hand indicated by the specific go cue (i.e. effector-specific motor activation) should be detectable. One way to assess effectorspecific motor activation is to analyze the lateralized readiness potential (LRP; e.g. Kutas and Donchin, 1980; for reviews see e.g. Eimer, 1998; Leuthold et al., 2004). When the response hand is specified in advance to stimulus presentation (e.g. by a pre-cue; Leuthold et al., 1996), the LRP may begin prior to the response signal thus indicating preparation of the correct response hand even before the target is presented. As a consequence, advance activation of the correct response hand in the specific go condition could be reflected in the LRP in response to the cue in this condition. This was indeed observed in the study by Smith et al. (2007; but see Bruin et al., 2001), thus further confirming the notion that the nogo P3 reflects the inhibition of hand-specific motor preparation. 1.3. Can motor inhibition account for the temporal orienting P3 effect?

1.2. The motor inhibition account on the nogo P3 There is an alternative explanation for the enhanced P3 to temporally attended non-targets, however: The temporal orienting P3 effect to non-targets may be related to processes associated with motor inhibition (see also Lampar and Lange, 2011), as has been suggested for the nogo P3 (e.g. Bruin et al., 2001; Karlin et al., 1970; Roberts et al., 1994). In go/nogo tasks, a P3 to nogo stimuli is frequently reported in addition to the P3 to go stimuli. The nogo P3 has a more anterior scalp distribution than the go P3 (e.g. Eimer, 1993;

The motor inhibition account on the nogo P3 offers an alternative explanation for the temporal orienting P3 effect for non-target stimuli. In the temporal orienting paradigm that was used in previous studies, stimuli are only relevant for response selection if they are presented at an attended time point (Lampar and Lange, 2011, Experiment 2; Lange et al., 2003, 2006). Participants may thus prepare a (specific) response only for the attended time point. Whether a response is actually executed depends on the type of stimulus presented (target or non-target). Because of the differential response

K. Lange / Biological Psychology 89 (2012) 433–443

preparation for attended and unattended time points, the participants need to actively withhold a response to non-targets only if the non-target is presented at an attended but not at an unattended time point, which is consistent with the notion that the observed temporal orienting P3 effect to non-targets reflects increased the increased inhibition of a planned movement for attended stimuli. 1.4. The present study The major goal of the present study was to test the motor inhibition account on the temporal orienting P3 effect to non-targets. In our earlier temporal orienting studies, the response hand was held constant over blocks of trials (e.g. Lange et al., 2003, 2006). Therefore, the attended and unattended conditions were conceptually equivalent to the “specific go priming” and “nogo priming” conditions of Bruin et al. (2001). However, because temporal orienting and response preparation did not vary independently in these studies, one cannot conclude that the effect was indeed due to the inhibition of a specific motor response. In the present study, both factors were therefore manipulated independently. The paradigm used here was a variant of the temporal orienting paradigm described above, with two major features changed. First, attention was varied within rather than between blocks, i.e. a cue at the beginning of each trial indicated which time point had to be attended. This was done to additionally test whether purely voluntary temporal attention can also be flexibly allocated on a trial-by-trial basis. E studies that reported early sensory modulations held the attentional focus constant for each block of trials (Lange et al., 2003, 2006; Lange and Röder, 2006; Sanders and Astheimer, 2008). Second, in the present study, two types of targets (piano and violin sounds) were used in addition to the non-targets (white noise bursts). The reason for this manipulation was to be able to vary the specificity of response preparation while holding constant physical stimulation. In the specific preparation condition, piano and violin tones required identical responses (simple reaction). Which response hand was used was constant for one half of the experiment. In the unspecific preparation condition, piano and violin tones required different responses (choice reaction). Thus, advance preparation of a specific response was only feasible in the specific preparation condition, which should mirrored in significant LRP activity prior to stimulus presentation in this condition, only (see also Smith et al., 2007).1 Stronger advance preparation of a specific response should call for more inhibition in order to withhold a response to an attended non-target stimulus in this condition. Therefore, if the temporal orienting P3 effect to non-targets is indeed related to the inhibition of a specific motor-response, the P3 should be larger to attended non-targets in the specific than in the unspecific preparation condition. This was the main hypothesis of the present study. Additionally, effects of temporal orienting were analyzed on both preparatory neural activity (elicited by attention-directing cues) and on the neural response evoked by non-targets.

1 The different response requirements were also associated with different perceptual demands, which were higher in the unspecific than in the specific condition. Whereas targets had to be distinguished from non-targets in both conditions, only the unspecific condition required a further discrimination between the different target stimuli. It is well-known that increasing the difficulty of the discrimination between targets and non-targets leads to a smaller P3 to targets (e.g. Fitzgerald and Picton, 1983; Hillyard and Kutas, 1983). This has been ascribed to the larger uncertainty of having correctly perceived the event (see also Ruchkin and Sutton, 1978). It is unlikely, though, that a similar pattern emerges for the ERPs to non-targets that were analyzed in the present study, since the discrimination of targets from nontargets did not differ between the response conditions: The decision that a stimulus is a non-target should be made with equal confidence in both conditions (see also Footnote 4).

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Preparatory activity encompasses both motor and sensory preparation and is indexed by a slow negative amplitude shift between the cue and the target, the contingent negative variation (CNV; Walter et al., 1964). Several processes contribute to this negativity: Stimulus anticipation (e.g. Walter et al., 1964), particularly when stimuli provide information with respect to the current task (Ruchkin et al., 1986; Van Boxtel and Brunia, 1994), time estimation (e.g. Macar and Besson, 1985; Ruchkin et al., 1977), and the preparation of a motor response (e.g. Birbaumer et al., 1990; see also Brunia, 1999). Previous temporal orienting studies have shown that the CNV over anterior and central scalp sites is more pronounced prior to the attended time point (e.g. Correa et al., 2006a; Lange et al., 2003; Sanders and Astheimer, 2008). Therefore, if temporal attention can be flexibly allocated on a trial-by-trial basis even when probability cuing is not involved, CNV should be larger prior to the attended time point. Moreover, both the N1 and the P3 evoked by non-targets at the end of short and long intervals should be larger in amplitude if the eliciting stimulus is presented at an attended time, as has been found previously (Lampar and Lange, 2011, Experiment 2; Lange et al., 2003, 2006; Lange and Röder, 2006; Röder et al., 2007). 2. Methods 2.1. Sample Thirty-four participants took part in the study. Three attended only the first of the two experimental sessions. Two participants were excluded from further analyses because they responded to less than two thirds of the attended targets or to more than one third of the unattended targets. One participant was excluded because more than one third of the trials per condition had to be removed from the EEG due to artifacts (Picton et al., 2000). The remaining sample consisted of 28 participants (20–34 years, mean 24 years). Seven participants were male, two were left-handed. None reported any hearing problems. Fifteen participants played or had played a musical instrument, but not at a professional level (1–11 years, mean 7 years). The study was performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki and approved by the ethical review board of the HHU Düsseldorf. All participants gave written informed consent and received course credit or a monetary compensation. 2.2. Stimuli and task Each trial consisted of an auditory cue, which was followed after 600 or 1200 ms by S2. The cue was either a high (880 Hz) or a low (220 Hz) triangle tone (200 ms, 10 ms rise/fall time), which was presented binaurally. S2 was either a white noise burst (100 ms), a violin tone (440 Hz, 100 ms), or a piano tone (440 Hz, 100 ms) and was presented either to the left or to the right ear. The noise burst was a non-target and thus always a nogo stimulus. The instrument sounds were the target stimuli and thus potential go stimuli, depending on whether or not they were presented at an attended time point (as detailed below). The triangle tone and the noise burst were generated with Adobe Audition 1.5. The instrument tones were generated with Cubase Studio 4, Version 4.5.2 (Steinberg). All auditory stimuli were presented via headphone (Koss 65). Cue stimuli were presented with a sound pressure level (SPL) of 77 db(A). The SPL of the noise bursts, the violin and the piano sounds was 67, 78, and 72 dB(A), respectively, so that these stimuli were of comparable loudness. To guide the participants’ attention to a particular time point, the cue indicated which time point (i.e. 600 or 1200 ms after cue onset) was relevant for response selection. Thus, this time point had to be attended. The participants were asked to respond if an instrument sound (target stimulus) was presented at the attended time point and to refrain from responding if an instrument sound was presented at the other (i.e. unattended) time point. Noise bursts were non-targets and thus never required any response. The assignment of the high or low cue-tone to the short or the long time interval was counterbalanced across participants. Participants responded to attended targets by pressing the left or right button on a custom made response box with their left or right index finger, respectively. Which response was required depended on the experimental condition. To vary the degree of possible preparation for a motor response, the two different targets either required identical (specific preparation condition) or different responses (unspecific preparation condition). In the specific preparation condition, both piano and violin sounds required, e.g., a right hand response in the first half of the blocks and a left hand response in the second half (the response hand changed after the first half of the experiment and participants always started with their preferred hand). In the unspecific preparation condition, the violin sound always required a left hand response and the piano sound always required a right hand response. In each condition, a trial ended as soon as a response was given or 1000 ms after S2. The inter-trial interval varied randomly between 500 and 1500 ms (rectangular distribution, mean 1000 ms).

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2.3. Procedure The experiment consisted of two sessions, one for each response preparation condition. The order of these conditions was counterbalanced across participants. In each session, participants were seated in an electrically shielded room. Written instructions presented on the computer screen informed the participants about the relationship between cue and S2 and which types of responses they should perform. The participants were asked to respond as fast and as accurately as possible to attended targets and to refrain from responding to unattended targets and all non-targets. To make the participants familiar with the task, several practice blocks were conducted. For both preparation conditions, the first practice block contained only trials with attended short intervals and the second practice block contained only trials with attended long intervals (first and second practice block: 32 trials each). The third and fourth practice block were identical to the experimental blocks except for the number of trials (third and fourth practice blocks: 32 trials each). The third and fourth practice blocks contained all experimental conditions, i.e. attended and unattended short and long intervals with equal probability. For the specific preparation condition, practice blocks were further subdivided and run separately for both response hands. Both for the practice blocks and for the experimental blocks, 50% of the trials contained a non-target and 50% of the trials contained a target (50% piano tone, 50% violin tone). The main experiment consisted of 6 blocks of 160 trials each. Every 40 trials, the participant could take a short break and received visual feedback about the performance in the past 40 trials via the computer screen (number of correct and incorrect responses and number of misses). Each testing session lasted about 2 h (including instruction, practice blocks, and application and removal of all electrodes). 2.4. Data analysis 2.4.1. Behavioral data: analysis Trials were only included in the analysis if reaction times were within two standard deviations (SD) of the participant’s mean reaction time in the corresponding condition (4.8% of the trials were rejected by this means; analyzing the uncorrected data yielded basically the same results). Dependent measures were reaction times, the percentage of correct responses to attended targets, and the percentage of erroneous responses to unattended targets (erroneous responses to non-targets were below 1% on average and were thus not examined further). Effects of interval and response preparation condition on the dependent measures were assessed with separate repeated measures analyses of variance (ANOVAs) with the factors interval (short vs. long) and preparation condition (specific vs. unspecific). 2.4.2. ERP data: recording The EEG was recorded from 31 ActiCap electrodes, i.e. using a high-impedance recording system (Brain Products, Gilching, Germany) inserted into an elastic cap from positions Fp1/2, F7/8, F3/4, Fz, FC5/6, FC1/2, FCz, T7/8, C3/4, Cz, CP5/6, CP1/2, TP9/10, P7/8, P3/4, Pz, O1/2, Oz. Eye movements were recorded with Fp1 (vertical eye movements) and F7 and F8 (horizontal eye movements). A ground electrode was placed at AFz. During recording, all electrodes were referenced to FCz. Impedances were kept at 25 k or below. The EEG was recorded continuously with a sampling rate of 250 Hz. The band-pass of the amplifiers (NuAmps, Neuroscan, Sterling, VA) was set from DC to 100 Hz. 2.4.3. ERP data: analysis Vision Analyzer 2 software (http://www.brainproducts.com) was used for ERP analyses. Off-line, electrodes were re-referenced to linked mastoids. Data were filtered with a Butterworth Zero Phase filter (0.01–30 Hz, 24 dB/oct). Whenever activity was less than 0.5 ␮V for a time epoch longer than 100 ms, the 200 ms before and after this event were removed. The ocular correction ICA function of Vision Analyzer 2 software was used to correct for blinks and horizontal eye movements. Both cue-related ERPs and ERPs elicited by non-targets were calculated. For the cue-related ERPs, segments started 200 before cue onset and ended 1800 ms after cue-onset (i.e. 600 ms after the end of the long interval). For the ERPs to non-targets, segments started 200 ms before target onset and ended 600 ms after target onset. Segments were removed according to the following criteria. The maximal allowed voltage step per sampling point was 50 ␮V. The maximal allowed absolute difference of two values in the segment was 80 ␮V, and the lowest allowed activity for 100 ms was 0.05 ␮V. If less than two thirds of the trials (i.e. less than 80 trials) remained in any one condition due to these criteria, the data of that participant were removed from further analyses (Picton et al., 2000). 2.4.3.1. Lateralized readiness potential. To test the assumption that the manipulation of response preparation condition indeed varied the degree to which a (specific) response was prepared in advance, the lateralized readiness potential (LRP) was calculated. Calculations were based on the cue-locked ERPs and were performed separately for the short and the long intervals and for the specific and the unspecific preparation conditions. For the unspecific preparation condition, the response hand is only defined when a target stimulus is presented. Therefore, only trials containing an attended target were used. This is justified, because only processes related to

response preparation prior to stimulus delivery were of interest here: Since participants did not know at the onset of a trial whether a target or a non-target was about to occur, processes associated with specific motor activation should be identical for target and non-target trials. Difference waveforms were calculated as follows: ERPC3 − ERPC4 for trials with a right hand response and ERPC4 − ERPC3 for trials with a left hand response. This eliminates activation symmetrical at both electrode sites. The resulting difference potentials were averaged to eliminate hemispheric differences unrelated to the response. In the resulting waveforms, negative amplitudes reflect activation of the correct response whereas positive amplitudes are related to activation of the incorrect response (for a review see Eimer, 1998; Leuthold et al., 2004). 2.4.3.2. Cue-related ERPs (CNV). To measure processes of temporal orienting in the CNV prior to the attended time point (e.g. Lange et al., 2003), ERPs to cues were averaged separately as a function of the to-be-attended interval (short or long) and the actual timing of the target (at the end of the short or long interval). Cue-related potentials were referenced to a 200 ms pre-cue baseline. The CNV was measured as the mean voltage over the 100 ms just preceding the ending of the short or long interval, respectively, i.e. 500–600 ms after the cue for the short interval and 1100–1200 ms after the cue for the long interval. 2.4.3.3. ERPs to non-targets (N1, P3). To measure effects of temporal orienting on non-target processing, ERPs to non-targets were averaged separately for the short and long intervals as a function of attention (attended vs. unattended). Trials were only included in the ERP if no response was given within the 1000 ms following the stimulus. Segments started 200 ms before the onset of the non-target or target stimulus and ended 600 ms thereafter. To minimize misalignment of the waveforms based on the CNV-activity, the baseline was set from 0 to 50 ms relative to the nontarget (Correa and Nobre, 2008; Lampar and Lange, 2011). Based on visual inspection of the grand average waveforms, N1 amplitudes were quantified as the mean voltage in the interval between 80 and 120 ms after onset of the non-target. P3 amplitudes were quantified as the mean voltage between 300 and 370 ms after onset of the non-target. Analyzing a later time window from 350 to 430 ms for the P3 yielded basically the same results. 2.4.4. ERP data: statistical analyses 2.4.4.1. Lateralized readiness potential. If participants used their pre-knowledge of the response hand, they should activate the correct response prior to the attended time point in the specific preparation condition. Moreover, because the timing of the possible target could be precisely predicted in this temporal orienting study, the pre-activation of the response should be strongly focused around the attended time point. Consequently, LRP-amplitudes should be significantly smaller than zero just prior to stimulus delivery for the specific condition. In the unspecific preparation condition, a pre-activation of the correct response-hand was not feasible. Therefore, the LRP should not start until after the presentation of the target and significant LRP activity should not be evident prior to target-onset. Thus, to assess the presence of motor preparation at the time point of target delivery, it was tested whether the LRP activity in the 100 ms prior to target onset significantly differed from zero using a t-test (for a similar procedure see Leuthold et al., 1996). This analysis served as a manipulation check for the response-preparation manipulation. To additionally test whether the onset of hand-specific motor preparation differed between the response-preparation conditions, the onset of the LRP was compared using the method suggested by Miller et al. (1998). An absolute criterion was used for determining LRP onset (−0.5 ␮V). Note, however, that using a relative criterion (30% of maximum amplitude) yielded basically the same results. 2.4.4.2. CNV and non-target N1 and -P3. For statistical analyses of the ERPs to the cue and to non-targets, electrodes F3, Fz, F4, C3, Cz, C4, P3, Pz, P4 were assigned one value of factor anterior–central–posterior (ACP) and one level of factor left–medial–right (LMR). The CNV and the N1 and P3 to non-targets were analyzed separately using repeated measures ANOVAs involving factors interval (short, long), preparation (specific, unspecific), attention (attended, unattended), LMR (left, midline, right), and ACP (anterior, central, parietal). Appropriate hierarchical ANOVAs were calculated to analyze higher-order interactions (O’Brien and Kaiser, 1985). For all analyses, the Greenhouse–Geisser correction was applied in order to compensate for violations of the sphericity assumption, where appropriate (Jennings and Wood, 1976). The corrected probabilities together with the corresponding ε values are reported. Partial 2 is reported as a measure of effect size. All analyses were conducted using SAS 9.2. For the ERPs, only effects involving factor attention are reported.

3. Results 3.1. Behavioral data Table 1 provides an overview of the behavioral data. Overall, participants responded to a high percentage of attended targets,

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Table 1 Mean percent correct to attended targets, percent false alarms to unattended targets, and reaction times to attended targets as a function of interval and preparation condition. The standard error of the mean is indicated in parentheses. Correct responses to attended targets

Erroneous responses to unattended targets

Reaction times

Specific

Unspecific

Specific

Unspecific

Specific

Unspecific

80.48 (1.41)

5.12 (1.26)

4.17 (0.95)

431 (16)

614 (11)

83.84 (1.45)

3.75 (0.86)

2.08 (0.42)

403 (15)

563 (12)

Short 89.05 (0.97) Long 87.32 (1.16)

but only rarely (less than 5%, on average) to unattended targets. Responses to non-target stimuli were below 1% on average. 3.1.1. Accuracy The percentage of correct responses was higher in the specific preparation condition than in the unspecific preparation condition (F(1, 27) = 33.82, p < .01, 2 = .56). The main effect of interval was not significant (F < 1). Because the main effect of preparation was qualified by an interaction with factor interval (Interval × Preparation: F(1, 27) = 14.05, p < .01, 2 = .34), separate analyses were conducted for the short and the long interval. The main effect of preparation was significant for both intervals (short: F(1, 27) = 44.94, p < .01, 2 = .62; long: F(1, 27) = 8.45, p < .01, 2 = .24), but the effect was larger for the short than for the long interval. False alarms to targets at unattended time points were rare overall, but participants responded more often to unattended targets at the end of the short than at the end of the long interval (Interval: F(1, 27) = 7.23, p = .01, 2 = .21). No other effects were significant. 3.1.2. Reaction times Participants responded faster in the specific preparation condition than in the unspecific preparation condition (Preparation: F(1, 27) = 210.01, p < .01, 2 = .89) and faster to stimuli at the end of the long interval (Interval: F(1, 27) = 43.07, p < .01, 2 = .61). A preparation by interval interaction was due to the fact that the main effect of interval was larger for the unspecific preparation condition than for the specific preparation condition (Preparation × Interval: F(1, 27) = 6.11, p = .02, 2 = .18; main effect Interval, specific: F(1, 27) = 12.92, p < .01, 2 = .32; unspecific: F(1, 27) = 46.72, p < .01, 2 = .63). 3.2. ERP data 3.2.1. LRP data In all conditions, an LRP was observed in the cue-locked ERP when a target stimulus occurred (Fig. 1). For both intervals, the LRP started earlier in the specific than in the unspecific preparation condition. For the short interval, the LRP crossed the criterion of −0.5 ␮V 630 ms after cue-onset (i.e. 30 ms after target onset) in the specific preparation condition and 904 ms after cue-onset in the unspecific preparation condition (i.e. 304 ms after target onset). The respective values for the long interval are 1334 ms after cueonset for the specific preparation condition (i.e. 134 ms after target onset) and 1426 ms after cue-onset for the unspecific preparation condition (i.e. 226 ms after target onset). t-Tests comparing the latencies of specific and unspecific conditions (Miller et al., 1998) confirmed that these differences were significant at the 5% level (short: t27 = 2.17; long: t27 = 0.92; both ps > .05). To check whether the response-preparation manipulation had the desired effect, the presence of effector-specific motor activation just prior to the attended time point was tested separately for the two intervals and preparation conditions. Significant responsespecific activity in the 100 ms preceding the attended time point was only observed for the specific preparation condition when the interval was short (specific: t27 = −2.09, p < .05, 2 = .14; unspecific:

Fig. 1. Grand-average waveform of the cue-locked LRP for trials with attended targets in the specific (solid line) and the unspecific (dotted line) preparation condition. The LRP is shown separately for short (top) and long intervals (bottom). The analysis epoch is highlighted in gray (500–600 ms and 1100–1200 ms post-cue for short and long intervals, respectively). The ending of the interval (presentation of S2) is marked by an arrow. Here, and in all figures, negativity is plotted up. Negative amplitudes signify preparation of the correct response hand.

t27 = 0.76, p = .45, 2 = .03). When the interval was long, significant activation was not observed in the 100 ms prior to target-onset (specific: t27 = 0.1, p = .93, 2 = .00; unspecific: t27 = −0.76, p = .37, 2 = .03).2 3.2.2. Contingent negative variation A CNV was observed between cue and S2 (see Fig. 2, left). In the 100 ms just preceding the end of the interval, this negativity was larger when the interval was attended (Attention, F(1, 27) = 40.14, p < .01, 2 = .60). When the interval was long but the short interval attended, negativity declined as soon as the ending of the short interval had passed (Fig. 2, bottom left). This caused a larger effect of attention for the long than for the short interval (Attention × Interval, F(1, 27) = 48.54, p < .01, 2 = .64; short: attention, F(1, 27) = 8.45, p < .01, 2 = .24; long: F(1, 27) = 55.40, p < .01, 2 = .67). Overall, the attention effect in the CNV had a central maximum, but the scalp topography of the effect differed slightly between

2 For the long interval, a visual inspection of earlier phases of the LRP suggested a possible pre-activation of the correct response hand in the unspecific preparation condition. To exclude this paradoxical effect, additional analyses were conducted for 100 ms time epochs between 600 and 1200 ms following the cue: Systematic response-specific activity was not observed.

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Fig. 2. Left: Grand Average waveforms (Cz), time locked to the attention-directing cue (indicated by a gray vertical line at time zero) in attended (solid lines) and unattended (dotted lines) conditions, separately for the short (top) and the long interval (bottom). Specific and unspecific preparation conditions are represented by thick and fine lines, respectively. The onset of S2 is marked by an arrow. ERPs are referred to a 200 ms pre-cue baseline. Right: Main effects of attention condition on the pre-target CNV (500–600 ms and 1100–1200 ms post-cue for short and long intervals, respectively) for single ACP and LMR clusters are presented as bar graphs, collapsed over preparation conditions (right; error bars represent the 95% confidence interval of the difference).

the short and the long interval (Attention × ACP: F(2, 54) = 12.66, p < .01, ε = .85, 2 = .32; Attention × ACP × Interval, F(2, 54) = 6.65, p < .01, ε = .70, 2 = .20; see also Fig. 2, right and Fig. 3, left). Whereas the central maximum of the attention effect was clearly supported by an Attention × ACP interaction for the long interval (F(1, 27) = 14.03, p < .01, ε = .80, 2 = .34), this interaction was not significant for the short interval (p > .16). Moreover, the attention effect was differently lateralized for short and long (Attention × LMR: F(2, 54) = 10.81, p < .01, ε = 1, 2 = .29; Attention × LMR × Interval, F(2, 54) = 19.77, p < .01, ε = .98, 2 = .42). For the long interval, the effect had its maximum over the midline (Attention × LMR, F(2, 54) = 18.49, p < .01, ε = .98, 2 = .41), whereas for the short interval, the effect was largest over the left scalp (Attention × LMR: F(2, 54) = 3.69, p = .03, ε = .96, 2 = .12). An Attention by Preparation Condition by ACP interaction was also observed (F(2, 54) = 5.68, p = .01, ε = .68; 2 = .17), but follow-up analyses showed that the Attention × Preparation interaction was not significant for either ACP cluster (frontal: p = .21; central and parietal: Fs < 1).

3 The present paper focuses on the ERPs to non-target stimuli for two reasons. First, the main hypothesis refers to the P3 to non-targets. Second, since attended

3.2.3. ERPs to non-target stimuli3 3.2.3.1. The N1 time range (80–120 ms). The overall ANOVA on mean amplitudes between 80 and 120 ms showed an interval by attention interaction (F(1, 27) = 7.16, p = .01, 2 = .21): Non-targets elicited a larger N1 when presented at an attended time point. Follow-up ANOVAs conducted separately for short and long intervals showed that this was only the case when the interval was short (Attention: F(1, 27) = 6.87, p = .01, 2 = .20; Attention × ACP: F(2, 54) = 3.81, p < .05, ε = .67, 2 = .12; long interval: effects involving factor Attention all ps > .28; see also Fig. 4). The effect of attention for the short interval had its maximum over the central scalp (Attention: frontal: p > .10, central: F(1, 27) = 11.24, p < .01, 2 = .29; parietal: F(1, 27) = 6.00, p = .02, 2 = .18; Fig. 3, top middle and Fig. 4A, right). Interactions between the factors attention and preparation were not observed (all ps > .12).

and unattended targets differ with respect to their response-assignment, attention effects in targets cannot be interpreted unequivocally. Note, however, that analyses of N1 and P3 to target stimuli yielded results that were very similar to those obtained for non-target stimuli, except that here the P3 and the temporal orienting P3 effect had a parietal maximum, which is consistent with their classification as go-stimuli (e.g. Eimer, 1993; Kok, 1986; Pfefferbaum et al., 1985; Roberts et al., 1994; Simson et al., 1977).

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Fig. 3. Topographic maps of the attention effects (ERPattended − ERPunattended ) observed in the CNV (left) and in the N1 (middle) and P3 (right) elicited by non-targets at the end of short (top) and long (bottom) intervals. CNV-effects are calculated based on cue-locked waveforms (500–600 ms and 1100–1200 ms post-cue for short and long intervals, respectively), relative to a 200 ms pre-cue baseline. Effects in N1 and P3 are calculated based on non-target ERPs (N1: 80–120 ms, P3: 300–370 ms) relative to a 50 ms post-stimulus baseline. Effects are shown separately for the specific and the unspecific preparation condition. Blue shadings reflect more negative amplitudes for the attended condition, red shadings more positive amplitudes for the attended condition. Darker shadings signal larger effects. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

To rule out the possibility that the N1 attention effect was due to residual differences between attention conditions carried over from the CNV, both attention effects were correlated for each electrode. Significant positive correlations, which could indicate a possible carry-over of the attention effect from the CNV to the N1, were not observed (all ps > .60). At Pz, the negative correlation tended to be significant (r = -.35, p = .07). 3.2.3.2. ERPs to non-target stimuli: the P3 (300–370 ms). The P3 to non-targets at attended time points was larger than the P3 to nontargets at unattended time points (main effect attention; exact statistical values are provided by Table 2; see also Fig. 4). This temporal orienting P3 effect was larger for the long than for the short interval (Attention × Interval) and larger for the specific than for the unspecific preparation condition (Attention × Preparation). The attention effect was maximally pronounced over the central scalp, but the exact topography was additionally modulated by interval and response preparation condition (see effects involving Attention and topographic factors in Table 2, see also Fig. 3, right). For the short interval, the attention effect was restricted to the central scalp (Attention × ACP: F(2, 54) = 5.95, p < .01, ε = .76, 2 = .18; central cluster: F(1, 27) = 4.86, p = .04, 2 = .15). For the Table 2 Results of the overall ANOVA on mean amplitudes of the P3 to non-target stimuli (300–370 ms). Only significant effects involving the factor attention are displayed. Effect

df

F

p

ε

2

Attention Attention × Interval Attention × ACP Attention × LMR Attention × ACP × LMR Attention × Interval × ACP Attention × Interval × LMR Attention × Preparation Attention × Preparation × ACP Attention × Preparation × ACP × LMR

1/27 1/27 2/54 2/54 4/108 2/54 2/54 1/27 2/54 4/108

56.98 108.5 24.37 26.11 2.75 6.66 52.07 7.55 17.38 3.22

<.01 <.01 <.01 <.01 .04 <.01 <.01 .01 <.01 .02

– –

.68 .80 .47 .49 .09 .20 .66 .22 .39 .11

.73 .91 .87 .80 .91 – .76 .84

long interval, the attention effect had a maximum over the central midline but was significant for all ACP and LMR clusters (Attention × ACP: F(2, 54) = 21.21, p < .01, ε = .77, 2 = .44; Attention × LMR: F(2, 54) = 57.57, p < .01, ε = .92, 2 = .68; Attention × ACP × LMR: F(4, 108) = 3.70, p = .01, ε = .89, 2 = .12; see also bar graphs in Fig. 4, right). For the specific preparation condition, the attention effect had its maximum over the central and frontal scalp, with a lateralization to the left (Attention × ACP: F(2, 54) = 21.52, p > .01, ε = .94, 2 = .44; Attention × LMR: F(2, 54) = 16.92, p > .01, ε = .71, 2 = .39; Attention × ACP × LMR: F(4, 108) = 5.57, p > .01, ε = .88, 2 = .17). For the unspecific condition, the attention effect had a central to parietal scalp distribution with a midline maximum (Attention × ACP: F(2, 54) = 23.27, p > .01, ε = .66, 2 = .46; Attention × LMR: F(2, 54) = 21.38, p > .01, ε = .94, 2 = .44). The main hypothesis of the present study was that the amplitude of the P3 to attended non-targets varies as a function of response preparation. To test this hypothesis, analyses were conducted separately for the attended and the unattended conditions (collapsed over factor interval, because significant interactions involving both factors interval and response preparation were not observed). For the attended condition, P3 amplitudes did not vary as a function of response preparation (effects involving factor response preparation: all ps > .15; see also Fig. 5, top). For the unattended condition, more specific response preparation led to smaller amplitudes of the P3 (main effect response preparation: F(1, 27) = 5.43, p = .03, 2 = .17; response preparation × ACP: F(2, 54) = 7.02, p < .01, 2 = .21). This effect had its maximum over the frontal scalp (frontal: F(1, 27) = 13.71, p < .01, 2 = .34; central: F(1, 27) = 4.12, p = .05, 2 = .13; parietal: F < 1; see also Fig. 5, bottom). 4. Discussion The present study manipulated both temporal orienting and the degree of hand-specific response preparation in a temporal trial-by-trial cuing paradigm. Attention effects both in preparatory activity reflected in the CNV and in processes related to different

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Fig. 4. Left: Grand Average ERPs elicited by attended (solid lines) and unattended (dotted lines) non-targets, separately for the short (A) and the long interval (B) and the specific and the unspecific condition. Electrodes C3, Cz, and C4 are displayed. Analyses epochs for both N1 (80–120 ms) and P3 (300–370 ms) time ranges are highlighted in gray. ERPs are referred to a 50 ms long post-stimulus baseline. The onset of the non-target is indicated by a gray vertical line. Right: Main effects of attention condition on the N1 and P3 for single ACP clusters are presented as bar graphs, collapsed over preparation conditions (error bars represent the 95% confidence interval of the difference).

levels of stimulus processing, reflected in the auditory N1 and the P3. This finding provides evidence that orienting purely voluntary temporal attention on a trial-by-trial basis leads to basically the same modulations of early and late processing steps has have been reported by earlier studies with a constant attentional focus (e.g. Lange et al., 2003, 2006). The primary goal of the present study was to test the motor inhibition account on the temporal orienting P3 effect to non-targets. Inconsistent with the assumption that the enhanced P3 to attended non-targets reflects increased motor

inhibition, P3 amplitudes to attended non-targets were not larger in the specific preparation condition, where the correct response hand was activated in advance. 4.1. Manipulation of temporal orienting and of response preparation A characteristic of the paradigm used in the present study is that attention effects cannot be analyzed at a behavioral level,

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Fig. 5. Left: Grand Average ERPs elicited at Fz by non-targets in the specific (dotted lines) and unspecific preparation task (solid lines), separately for the attended (top) and unattended condition (bottom) and the short (right) and long (middle) interval. The analysis epoch for P3 (300–370 ms) is highlighted in gray. ERPs are referred to a 50 ms long post-stimulus baseline. The onset of the non-target is indicated by a gray vertical line. Right: Main effects of preparation condition on the P3 for single ACP clusters are presented as bar graphs, collapsed over short and long intervals (error bars represent the 95% confidence interval of the difference).

because overt responses are restricted to attended stimuli. Analyses of behavioral data therefore serve mainly to make sure that participants followed the task instructions. The present results do indeed show that the participants responded to a high proportion of targets at the attended time but hardly ever to targets at the unattended time or to non-targets. Consistent with earlier findings, the temporal orienting of attention was also reflected in the CNV following the cue: For both time intervals, negativity was larger prior to the attended time point. That the CNV is indeed related to temporal orienting can be seen most clearly in the case of the unattended long interval: Here, negativity declined almost to baseline levels after the ending of the (attended) short interval had passed (see also Correa et al., 2006a; Griffin et al., 2002; Lampar and Lange, 2011, Experiment 2; Lange et al., 2003; Miniussi et al., 1999; Sanders and Astheimer, 2008). The CNV in temporal orienting studies has been associated with processes controlling the allocation of attention in time (e.g. Correa et al., 2006a; Lange et al., 2003). It has been shown that cognitive preparation is reflected by a frontally distributed CNV, whereas the CNV related to motor preparation has a more central topography (Falkenstein et al., 2003; Leynes et al., 1998). The temporal orienting CNV may thus encompass both processes related to the allocation of sensory processing resources to the attended time point and motor preparation. The relative contribution of these processes has to be investigated by future research. The participants either did (specific preparation) or did not know the response hand before the target was presented (unspecific preparation). They should thus have been able to activate the appropriate response hand prior to the attended time point in the specific preparation condition. LRP data confirmed this assumption, at least for the short interval: Significant activation of the correct response hand prior to the attended time point was only observed when the response hand was known in advance (see also Leuthold et al., 1996; Smith et al., 2007). For the long interval, the LRP started only after the target was presented for both specific and unspecific preparation. This pattern suggests that participants used different response-preparation strategies when attending the short versus

the long interval. For both intervals, however, the LRP started earlier in the specific than the unspecific preparation condition, suggesting that participants actually did prepare the correct response hand earlier when specific preparation was feasible. 4.2. The P3 to non-targets – an index of motor inhibition? As expected and consistent with earlier findings, temporally attended non-targets elicited a larger P3 than temporally unattended non-targets (Lampar and Lange, 2011; Lange et al., 2003, 2006). Both the P3 and the temporal orienting P3 effect for nontargets had a central maximum; which is in line with that described earlier for the nogo P3 (e.g. Eimer, 1993; Karlin et al., 1970; Kok, 1986; Pfefferbaum et al., 1985; Roberts et al., 1994; Simson et al., 1977). According to the motor inhibition account, the nogo P3 should be larger when motor inhibition is required, e.g. when a specific motor response was pre-activated (Bruin et al., 2001; Smith et al., 2007), Importantly, the LRP confirmed advance motor activation in the specific, but not the unspecific, preparation condition, at least for the short interval. Thus, if the temporal orienting P3 effect to non-targets reflects motor inhibition, the P3 to attended nontargets should be larger in the specific condition (see also Bruin et al., 2001; Smith et al., 2007). Inconsistent with this notion, however, the P3 to attended non-targets of short intervals did not differ between preparation conditions. That the temporal orienting P3 effect was affected by preparation condition was due to a smaller P3 for the specific than the unspecific condition for unattended non-targets.4 Thus, the present findings do not support the

4 In light of this finding, it seems all the more unlikely that the effect of response preparation is rather an effect of a more difficult perceptual task in the unspecific condition (see Footnote 1). Even if one assumed that the same effect was observed for non-targets as for targets, increased task difficulty is known to lead to a smaller P3 (e.g. Fitzgerald and Picton, 1983; Hillyard and Kutas, 1983) By contrast, in the present study, P3 to unattended non-targets was smaller for the specific (i.e. the easier) condition.

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hypothesis that processes related to the inhibition of a specific motor response play a major role in the generation of the temporal orienting P3-effect to non-targets. The nogo P3 may also be related to the evaluation of the outcome of inhibition (Beste et al., 2008, 2010; Dimoska et al., 2003, 2006; Fallgatter et al., 2004; Roche et al., 2005) rather than inhibition per se. This interpretation may also explain the temporal orienting P3-effect to non-targets, since evaluation should be necessary only for the time point at which a response may be required. The present study was not designed to test this idea. However, regarding the nogo P3 as an index of the evaluation of inhibition does not provide a straightforward explanation why a smaller P3 was observed in the specific than the unspecific preparation condition for unattended non-targets. 4.3. P3 and information delivery The exact functional significance of the temporal orienting P3 effect to non-targets remains elusive. While response inhibition does not offer a suitable explanation for the present result pattern, the P3 may reflect the different amounts of information conveyed at attended and unattended endings of short or long intervals, as suggested earlier (Lange et al., 2003). In the temporal orienting task used in the present study, the most relevant information is provided at the attended time point, hence the larger P3 to attended stimuli. For the unattended time point, whether stimulus presentation provides information for the participant differs for short and long intervals: Relevant information is only provided at the end of the short interval. Consistent with the idea that P3 amplitude reflects the amount of information provided, a marked P3 was recorded to unattended non-targets of the short interval, but there was practically no P3 in the unattended long condition, similar as in our previous research (e.g. Lange et al., 2003). The pattern found for P3 amplitudes is very similar to that observed in the CNV: Whereas a negativity was clearly present prior to the ending of the unattended short interval, the CNV was almost at baseline levels at the offset of the unattended long interval (Fig. 2, bottom; see also Lange et al., 2003). An association between these two ERP effects was further corroborated by exploratory correlations between the attention effects observed in the CNV and in the P3. It has been suggested earlier that the positivity in the time range of the P3 is due to the resolution of pre-target negativity/CNV (e.g. Deecke et al., 1984; Deecke and Lang, 1988; Karlin, 1970; Näätänen, 1970; but see Donchin and Heffley, 1979; Donchin et al., 1975). It thus remains to be investigated whether the temporal orienting P3 effect is an epiphenomenon resulting from differential CNV resolution or reflects a functionally distinct process that happens to be sensitive to some of the factors influencing CNV. Notably, both the P3 and the CNV have been related to information delivery: The P3 is sensitive to the information that is actually delivered (Johnson and Donchin, 1978; Ruchkin and Sutton, 1978; Sutton et al., 1965, 1967), while the CNV reflects the anticipation of information (Ruchkin et al., 1986; Van Boxtel and Brunia, 1994). Because in temporal orienting the information content of a stimulus depends crucially on its temporal position, similar effects may be observed in the CNV and the P3. Note, however, that the different amounts of information provided does not completely explain P3 amplitudes, because these were larger for attended long than attended short intervals, whereas more information is provided at the ending of the short interval (see above). 4.4. The temporal orienting N1 effect does not require a constant attentional focus A second goal of the present study was to explore whether effects of purely volitional temporal attention generalize to experimental conditions, where attention varies between trials (variable

focus of attention) rather than between blocks (constant focus of attention; Lange et al., 2003, 2006). The present enhancements of the CNV and of the N1 and P3 suggest that this is indeed the case, at least for the short interval. To make sure that the larger N1 to temporally attended stimuli reflects an enhancement of early, perceptual processing stages, the N1 attention effect has to be disentangled from attention effects in the preparatory negativity (see e.g. Lange et al., 2003). The present study approached this problem by using a conservative post-stimulus baseline (see also Correa and Nobre, 2008; Lampar and Lange, 2011) and by showing that the two effects were not correlated. It remains to be investigated, why an enhanced N1 for longinterval non-targets was not observed in the present study, whereas we did find such an effect in earlier studies (Lange et al., 2003, 2006; Lange and Röder, 2006; see also Sanders and Astheimer, 2008). Nevertheless, the enhancement of the N1 for the short interval is an important finding, because it has been hypothesized earlier that early, sensory effects of temporal attention may depend on a constant focus of attention (Correa et al., 2006a; Lange and Röder, 2006) The present data suggest, that this is not crucial per se, at least in the auditory modality (see also Lampar and Lange, 2011, Experiment 2), but a direct comparison of both conditions is necessary to draw firm conclusions. So far, only few studies have directly compared attention effects for a variable versus constant attentional focus. In visual spatial attention, behavioral (Posner et al., 1980, Experiment 1) and ERP effects are larger with a variable attentional focus (Eimer, 1996). By contrast, in visual temporal orienting, larger behavioral effects were found when attention was constantly oriented to the same time point (Correa et al., 2004, 2006b). To the best of my knowledge, it has not been investigated so far whether a variable versus constant attentional focus has a similar effect on early ERP effects of temporal orienting. This needs to be addressed by future studies.

5. Conclusion The findings of the present study both replicate and extend previous research on auditory temporal orienting. As in earlier studies, an enhancement of both early (i.e. the auditory N1) and late (the P3) processing stages was observed. Earlier findings were extended by showing that the N1 effect of purely volitional temporal orienting is also found when attention varies between trials. Most importantly, it was shown that the enhanced P3 to attended non-targets does – most likely – not reflect increased response inhibition, although the P3 effect was larger when advance response activation was stronger. While the functional significance of the P3 effect to nontargets remains to be elucidated, it may be related to the amount of information delivered at attended and unattended time points of short and long intervals.

Acknowledgments The study was supported by the Deutsche Forschungsgemeinschaft (German Research Foundation), Grant LA 2486/1-2. I thank Selina Hasse, Alexa Lampar, Carina Kreitz, Robert Schnürch and Lukas Verfürden for their help during data acquisition and Daniela Czernochowski and Martin Heil for helpful comments.

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